.\" -*- nroff -*-
.\" Copyright 2013 Los Alamos National Security, LLC. All rights reserved.
.\" Copyright 2010 Cisco Systems, Inc.  All rights reserved.
.\" Copyright 2006-2008 Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines Corporation
.\" $COPYRIGHT$
.TH MPI_Comm_create_group 3 "Jun 10, 2020" "4.0.4" "Open MPI"
.SH NAME
\fBMPI_Comm_create_group\fP \- Creates a new communicator.

.SH SYNTAX
.ft R
.SH C Syntax
.nf
#include <mpi.h>
int MPI_Comm_create_group(MPI_Comm \fIcomm\fP, MPI_Group\fI group\fP, int\fI tag\fP, MPI_Comm\fI *newcomm\fP)

.fi
.SH INPUT PARAMETERS
.ft R
.TP 1i
comm
Communicator (handle).
.TP 1i
group
Group, which is a subset of the group of comm (handle).
.TP 1i
tag
Tag (integer).

.SH OUTPUT PARAMETERS
.ft R
.TP 1i
newcomm
New communicator (handle).
.ft R
.TP 1i
IERROR
Fortran only: Error status (integer).

.SH DESCRIPTION
.ft R
MPI_Comm_create_group is similar to MPI_Comm_create; however,
MPI_Comm_create must be called by all processes in the group of
comm, whereas MPI_Comm_create_group must be called by all processes in group,
which is a subgroup of the group of \fIcomm\fP. In addition, MPI_Comm_create_group
requires that \fIcomm\fP is an intracommunicator. MPI_Comm_create_group returns a new
intracommunicator, \fInewcomm\fP, for which the group argument defines the communication
group. No cached information propagates from \fIcomm\fP to \fInewcomm\fP.

Each process must provide a group argument that is a subgroup of the group associated with \fIcomm\fP;
this could be MPI_GROUP_EMPTY. If a non-empty group is specified, then all processes in that
group must call the function, and each of these processes must provide the same arguments,
including a group that contains the same members with the same ordering. Otherwise
the call is erroneous. If the calling process is a member of the group given as the \fIgroup\fP
argument, then newcomm is a communicator with group as its associated group. If the
calling process is not a member of group, e.g., \fIgroup\fP is MPI_GROUP_EMPTY, then the call
is a local operation and MPI_COMM_NULL is returned as \fInewcomm\fP.

.sp
.LP

.SH NOTES
MPI_Comm_create_group provides a means of making a subset of processes for the purpose of separate MIMD computation, with separate communication space. \fInewcomm\fR, which is created by MPI_Comm_create_group, can be used in subsequent calls to MPI_Comm_create_group (or other communicator constructors) to further subdivide a computation into parallel sub-computations. A more general service is provided by MPI_Comm_split.

.SH ERRORS
Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument.
.sp
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

.SH SEE ALSO
.ft R
MPI_Comm_create

